Stainless steel composition



United States Patent 3,071,460 STAENLESS STEEL COMPOSITION Harry Tanczyn, Baltimore, Md., assignor to Armco Steel Corporation, a corporation of Ohio No Drawing. Filed Nov. 20, 1959, Ser. No. 854,227 5 Claims. (Cl. 75-124) My invention relates to the stainless steels, more particularly a heat-hardenable chromium-nickel-aluminum stainless steel, a method of heat-treating the same, and the resulting hardened stainless steel itself.

One of the objects of the invention is the provision of a chromiumnickel-aluminum stainless steel which is comparatively soft and workable in one condition of heattreatment and which readily may be hardened for duty by treatment at temperatures which are not unduly high.

A further object is the provision of such a steel and method of treating the same to achieve a hard, strong and tough steel, a steel having excellent yield strength as Well as ultimate tensile strength,, together with good ductility and good impact strength.

A still further object of my invent-ion is the provision of a chromium-nickel-aluminum stainless steel which lends itself to hot-working as by forging, rolling and piercing, into a variety of flat, round or special shapes, or tubes, for fabrication and heat-treatment to yield a host of articles and products of ultimate use made to desired specification and tolerance with a minimum loss of metal in treatment. 7

Another object is the provision of a steel of the character noted which readily may be made to desired specification with a minimum of alloying ingredients and at minimum cost.

Other objects of my invention in part will be obvious and in part pointed out in the course of the following specification.

My invention accordingly consists of the combination of elements and relative proportioning of the same, as well as in the various heat-treating steps and the relation of each of the same to one or more of the others, particularly with reference to the combination of elements, and in the final heat-treated product, all as described herein and particularly set forth in the claims at the end of this specification.

In order to provide a better understanding of certain features of my invention it may be noted at this point that the stainless steels generally are defined as those steels which include about 10% to 35% chromium, with or without nickel, and with or without additions of any one or more of a number of ingredients employed for special purposes. These ingredients commonly include manganese, silicon, copper, cobalt, molybdenum, aluminum, tungsten, vanadium, titanium, colurnbium, and the like. The carbon content usually is low, although, for special purposes, it may be substantial.

Of the many grades of stainless steel now on the market it generally is considered that it is the chromium-nickel grades which possess the best working and forming characteristics. These steels readily may be hot-Worked and cold-worked. They may be bent, pressed, stamped, punched and machined to form a host of useful articles, products and component parts. And many of the chromium-nickel stainless steels are greatly benefited through the addition of one or more of the strong carbide-forming elements titanium and columbium. Others are benefited through the addition of copper. And in still others through the addition of aluminum.

The chromium-nickel-aluminum stainless steels presently available are not only comparatively soft, ductile and readily workable in one condition of heat-treatment, but they are easily hardened by heat-treatment to give a steel which is of great strength. In many applications,

however, it is found that these steels suffer in ductility and impact strength in the heat-hardened condition. And frequently it is found that there is a loss of corrosion resistance, this being particularly felt in the steel as a part of the complete fabricating and final heat-treating operation.

Accordingly, one of the objects of my invention is the provision of a particular chromium-nickel-aluminum stainless steel which possesses the many desirable characteristics of the known chromium-nickel-aluminum stainless steels in matters of formability and subsequent hardena-bility, together with improved hot-working characteristics at the steel mill in the production of plate, sheet, strip, bars, rods, wire and tubes, as well as improved resistance to corrosive attack of common pickling solutions employed by the mill, or by the fabricator, and which steel, in addition to possessing the strength of the prior chromium-nickel-aluminum steels in heat-hardened condition, possesses greatly improved ductility, impact strength and the ability to withstand sudden stresses along various axes without crack or failure.

Referring now to the practice of my invention, I provide a chrornium-nickel-aluminurn stainless steel of substantial nitrogen content and minimum carbon content. The chromium content of my steel amounts to about 14.00% to 21.00%, the nickel content from 2.50% to 8.00%, the aluminum content from .70% to 2.50%, the nitrogen content from .10% to .40%, a manganese content of up to 8%, with the manganese content being inversely proportioned with respect to the nickel content, and the carbon content not exceeding .04%, with remainder substantially all iron. The proportiom'ng of these several ingredients is in every sense critical. Where the chromium content is less than 14.00%, the corrosion resistance characteristics seriously sulfer and where it exceeds 21.00%, I find that inadequate hardening is achieved. A nickel content of anything less than 2.50% gives a steel which is objectionably hard in the annealed condition with consequent loss of forming properties. With a nickel content exceeding 8.00%, I find that the steel inadequately responds to heat-treatment and fails to develop the hardness desired in the final heat-hardened condition. Much the same may be said for the aluminum content, for with an aluminum content less than .70% inadequate hardening is bad and where it exceeds 2.50% the steel sufiers a loss of hardenability; it does not harden to the full extent.

In the steel of my invention the carbon content should not exceed 04% because with a higher carbon content I find a definite loss of corrosion resistance, especially in pickling the steel as with the known nitric acid-hydro fluoric acid pickling solutions commonly used to remove heat-tint resulting from the heat-treating operations referred to hereinafter. Actually, I much prefer that the carbon content shall not exceed 0.03% in order to positively assure freedom from corrosive attack. And in the production of extremely thin metal sections, sheet and strip on the order of some .005 to .010" as for the aircraft industry, I prefer a carbon content not exceeding .025 I find that where the carbon content is unduly high there is a substantial loss of metal, a loss which cannot be tolerated in thin metal sections.

In the steel of my invention nitrogen is an essential ingredient. For I find that nitrogen lends hardness to the steel in final heat-treated condition, this in many respects being like the effect of carbon. Surprisingly enough, it also contributes to ductility and impact strength. Where a lesser amount of nitrogen is employed, that is, less than .l0%, the high ductility and impact strength of my steel is lost. And where it substantially exceeds .40% the composition balance is adversely alfected, and the steel is inclined to become objectionably hard in the annealed condition, all with loss of Working and forming properties.

In the steel of my invention the silicon content amounts to about 2.00% as a maximum, the phosphorus content a maximum of .050% and the sulphur content likewise a maximum of .050%. Manganese, of course, is present, this in amounts up to 2.00% as a maximum. Where desired, however, the nickel content of my steel may be partially replaced by manganese on the usual 2 for 1 basis, that is, 2% manganese for every 1% of nickel replaced. But I find that there must be at least 2.50% actual nickel content. And that the total manganese content, this including the amounts commonly present and the amounts employed for replacing the nickel, shall not exceed 8%. A manganese content of 8% is used with a nickel content of 2.5%. And with increased nickel contents the manganese content is proportionately less, this on down to a maximum of 2% Where the nickel content amounts to 8.00%. With an excessive amount of manganese there is a loss of ductility and impact strength in the final heat-hardened condition.

Where desired, I include in the composition of my steel a small amount of copper, this up to 5% as a maximum. Copper apparently aids in hardening the steel, although an amount exceeding 5% is objectionable in that hotworkability suffers. And of even greater significance the Welding characteristics of the steel substantially suffer with an excess amount of copper.

The steel of my invention therefore essentially consists of chromium in the amount of 14.00% to 21.00%, nickel in the amount of 2.50% to 8.00%, with manganese 8% when the nickel is 2.5% and not exceeding 2% When the nickel content is 8.00%, aluminum in the amount of .70% to 2.00%, nitrogen in the amount of .10% to .40%, carbon .04% maximum, preferably 03% maximum, and more particularly a maximum of .025%, with silicon 2.0% maximum, phosphorus and sulphur each .050% maximum, and copper a maximum of 5.00%, with the remainder of the steel substantially all iron. The preferred steel of my invention as appears more fully hereinafter analyzes 16.5% to 17.5% chromium, 6.0% to 7.5% nickel, .8% to 1.5% aluminum, .10% to .40% nitrogen, carbon 03% maximum, manganese 1.5% maximum, silicon 1.0% maximum, with the remainder substantially all iron.

The steel of my invention is melted in accordance with the procedures generally described and claimed in one or more of the US. Letters Patent 1,925,182 of September 5, 1933, issued to Alexander L. Feild and entitled Process for the Manufacture of Rustless Iron, Patent 2,455,073, issued November 30, 1948, to Donald L. Loveless and entitled Production of Stainless Steel, and Patent 2,621,119 of December 9, 1952, issued to Donald 'L. Loveless and entitled Stainless Steel Melting Process.

The required nitrogen content is conveniently introduced in the manner described in the Weitzenkorn U.S. Patent Process and Product or for the steels of high manganese,

in accordance with the method described in my US.

Patent 2,696,433 of December 9, 1954, and entitled Production of High Nitrogen-Manganese Steel.

The steel in the form of ingots is fashioned into blooms and billets and thence converted into plate, sheet, strip, bars, rods, wire and tubes at the mill. I find that it pos- Sesses improved hot-working characteristics, this as compared with the prior chromium-nickel-aluminum stainless steels.

The various converted products are annealed at a temperature of 1800 to 2000 F. and either cooled in air 01' quenched in water, as desired. The time at annealing temperature does not appear to be particularly critical, usually one-half hour at temperature being sufficient. The steel in the annealed condition is substantially fully austenitic. The metal is soft and ductile, the hardness not exceeding Rockwell B90, It is readily worked and formed as by bending, drawing, spinning or machining (punching, drilling, cutting or the like) to produce a variety of articles, products and parts for industry and commerce. It is readily fashioned into various structural members required for airplanes. It is easily made into bolts, screws and nuts. It is suitably fashioned into valves and valve parts, die blocks and surgical instruments. In the working, forming and fabrication of the steel particular advantage is taken of its excellent workability and formability in the annealed condition. It will be recognized, of course, that these various working, forming and fabricating operations are usually performed by the customer-fabricator.

Following fabrication the various articles, products and parts are preliminarily hardened by heating the same, preferably in a suitable heat-treating furnace, to a temperature of about 1400 F. for a sufficient length of time to effect a precipitation of the carbides present in the steel, presumably carbides of chromium. There also may e some precipitation of aluminum compounds. The transformation point of the austenitic matrix appears to be raised somewhat. And the steel, upon cooling, as by water-quench, transforms into martensite or a martensite-like constituent with some free-ferrite present. The steel in this condition, for a typical example, has a hardness amounting to some Rockwell C 22-28.

The high nitrogen chromium-nickel-aluminum stainless steel of my invention retains sutficicnt softness and ductility in the preliminary hardened condition to permit working as by cold-rolling and drawing, and machining as by turning, cutting, punching, drilling and the like, so that where desired one or more of the various working, forming and fabricating operations referred to above may be delayed and practiced on the preliminary hardened steel rather than upon the annealed metal coming from the mill. Actually, where the production of parts made to critical tolerances are required, I find advantage in working with the annealed and preliminarily hardened steel plate, sheet, strip, bars, rods, wire and the like for the reason that the fabricated products need not be subjected to phase transformation, and possible slight change in dimension, which may be encountered in preliminarily hardening of the stainless steel parts produced from the annealed steel.

In the further practice of my invention I find that as a result of the preliminary hardening treatment at a temperature of some 1400 F. there is formed on the various stainless steel articles, products and parts a light heat-tint. This is of course in an oxide film of significant dimension. It is removed by pickling the various products in an aqueous solution of nitric and hydrofluoric acids. In general, the nitric acid content amounts to about 15% by volume and the hydrofluoric acid about 2%, with remainder water. I find that my steel is possessed of superior corrosion resisting properties and that even the thinnest sections, that is, sections on the order of .005 to .010", are reliably pickled without fear of metal loss or penetration through the metal as occasionally encountered in the steels of the prior art. My steel in no way suffers from intergranular attack. This I theorize is due to the employment in my steel ofbut a minimum of carbon with the consequent formation of a bare minimum of carbides.

The preliminary hardened chrominum-nickel-aluminum stainless steels of high nitrogen content and low carbon content are now given a final hardening treatment by reheating at a temperature of about 900 F. for about an hour. Actually, the temperature may range from some 750 to 1100 F. for a time of half an hour at the higher temperature to some 2 hours or more at the lower temperature, but I find that a temperature of 900 F. gives excellent results. With this final hardening treatment and cooling in air or water the martensite or martensite-like constituent of the preliminarily hardened steel is fully retained.

While I do not care to be bound by explanation, it is my view that in the final hardening treatment there is precipitated an aluminum compound, very likely a compound of nickel and aluminum, which gives rise to the further hardening had in the final heat-treated condition. It may be that the reaction involves some rearrangement or reordering of the lattice structure of the aluminum compound within the lattice structure of the austenitic matrix which gives rise to the hardening effect. However all that may be, the steel of my invention, as finally hardened by heat-treatment, is hard and strong and yet ductile and tough. The hardness had amounts to some Rockwell C 41 to 46, all as more fully dealt with hereinafter.

As specifically illustrative of the practice of my invention I have prepared two samples of chromium-nickelaluminum stainless steels of high nitrogen content and low carbon content, the chemical analyses of which are given in Table I below:

TABLE I Chemical Analyses of Cr-Ni-Al Stainless Steels of High Nitrogen Content and Low Carbon Content 6 The aluminum content of the steel appears to remain in solution even after the quenching or cooling.

In the preliminarily hardened condition of my steel the austenite has transformed to martensite, or a martensitelike constituent. 1 find that there are also a few scattered carbides at the ferrite boundaries and also a small amount of retained austenite. Moreover, some carbides appear to be located at the borders of the secondary austenite.

With final hardening themartensite or martensite-like constituent is fully retained and so, too, the free-ferrite with a few scattered carbides at the ferrite boundaries. It appears, however, that the secondary austenite as Well as the retained austenite is now transformed to martensite and that the carbides previously referred to are also located at the positions formerly occupied by the secondary austenite, that is around the free-ferrite.

The mechanical properties of the typical steel of my invention (Heat E8826) is given below in Table III:

TABLE III Mechanical Properties of the Heat E8826 0 Table I in Heat C MD P 8 Si Cr Ni Al N Cu the Prelzmlnarlly Hardened and m the Flnal Hardened N0, Condition E8820 .028 17.20 6.98 .94 .096 U.'1.S., 0.2% Percent Percent Rock. Izod Ess56 .023 1.14 .009 .012 17.35 7.01 1.07 .14 Condition p.s.i. .s., E in 2" red. hard. 'mpaet,

p.s.i. area ft./lbs. The samplesof my stainless steel as illu'stratively set Preham (19000 forth above were annealed at a temperature of 1900 F. g Q for a period Of one-half hour andthen quenched 111 Wal'fil'. \I V QZ) 125 0 84 000 14 5 50 0 027 25/2 Following annealing and quenching they were heated at g ag g g g a temperature of 1400" F. for a period of 90 minutes and X j 181,200 163,200 1 4 0 043 0/9 water-quenched, after which they were reheated at 900 F. for 1 hour and cooled in air. The hardness had in the various conditions of annealing, preliminary hardening and final hardening are given in Table II below:

TABLE II Hardness Values of the Sample Steels of Table I in the {her heat of Stainless Steel i my invention Annealed preliminarily Hardened and Final (Heat E9018) as compared wlth the low nitrogen steel of Hardened Conditions the prior art (Heat E9026). The chemical analyses of the two steels is given in Table IV below with corresponding mechanical properties in Table V in the annealed conig jgyig fg ggglfim t g 4.5 dition 1700 F., also 1900 F.), the preliminary hardened 90 min. WlQl) 1 hr. no.) condition, and the final hardened condition:

E8826." Rockwell 1386/81. Rockwell C27... Rockwell 043. TABLE IV E8856 Rockwell B88 Rockwell Chemical Analyses of Two CrNi-Al Stainless Steels 5O structurally, a typical illustrative steel of my invention Heat 0 Mn P S S or Ni Al N (E8826) 18 found to be substantially fully austenltlc 1n the annealed condition, with any free-ferrite not exceeding E9018 .032 .012 1683 131 about 20% by volume. It appears that the free-ferrite E9026 17-03 present is rimmed with what I call secondary austenite.

TABLE V Comparative Mechanical Properties of the Steels of Table IV in the Annealed, Preliminarily Hardened and the Final Hardened Conditions of Heat-Treatment Heat U.T.S., 0.2% Percent Percent Rock Izod N0. Condition p.s.i. Y.S., E in red. hard impact p.s.i. 2 area ftJlbs.

{Annealed 1,700 F. 1,700 F. hr. we. {Pm hard. 1,700 F. hr. We. +1,400 F. 00 min. W.Q.). gig, 130018-. {Finally hard. (1,700 F. 4 hr. W.Q. 1,400F. min. 15-20 120020" W.Q. +950 F. hr. A.O.). 1 0% to 7% {Annealed 1,000 F. 1,900 F. hr. W.Q.) I 621% {Pre. hard. 1,000 F. hr W.Q. +1,400 F. 90 min. W.Q.) E90ls {Finallyhard (1,000 12% hr.W.Q. +1,400 F. 90 mih.W.Q. 184, 800 15% to is E9026 +950 F. 1 hr. A.C.). 203, 200 3% t0 4% It will be seen from Table V above that the steel of my invention (E9018) with carbon content on the order of 03%, is characterized by high hardness, good strength, and yet excellent ductility and toughness. In finally hardened condition, from an anneal at 1700" F., the preferred steel of my invention has an Izod impact strength of 1520 ft. lbs. While that of the prior art (Heat E9026) has an impact strength of only some 6 /2 to 7 /2 ft. lbs. In the finally hardened condition from an anneal of 1900 F. my steel has an impact strength of 15 /2 to 18 ft. lbs. while the impact strength of the steel of the prior art only amounts to 3 /2 to 4 /2 ft. lbs. The elongation figures for my steel are appreciably in excess of the figures had in the steel of the prior art. Although the figures for strength are in favor of the prior art steel this in no way offsets the much greater toughness had with the steels of my invention.

Thus it will be seen that I provide in my invention a chromium-nickel-aluminum steel in which there are had the various objects hereinbefore set forth, together with many practical advantages. The chromium-nickel-aluminum stainless steel of my invention of high nitrogen content and low carbon content works well in the mill, is of improved corrosion resistance and readily lends itself to forming and fabricating by well known methods of bending, spinning and drawing, as well as punching, cutting, drilling, and other machining in the production of a wide variety of articles, products and component parts. The steel of my invention may be brazed and welded by known methods in fabricating the various products of use. Those articles, products and component parts are readily heat-hardened by double heat-treatment, that is, preliminary hardening treatment followed by a final hardening treatment, at comparatively low temperatures. The finally hardened steel not only has good yield strength and ultimate tensile strength, but possesses good ductility and toughness, the toughness greatly exceeding that of chromium-nickel-aluminum stainless steels of the prior art. The steel of my invention is peculiarly adapted to the construction of tanks and pressure vessels in that the greater ductility permits greater stresses without risking rapid crack propagation and failure under stress. The strength and toughness of my steel lies not only in the direction of rolling or other working but in the transverse direction as well.

Inasmuch as many possible embodiments may be made of my invention, and many changes made in the embodiments herein set forth, it is to be understood that all matter described herein is to be interpreted as illustrative and not as a limitation.

I claim as my invention:

1. A chromium-nickel-aluminum stainless steel susceptible to precipitation-hardening by double heat-treatment from a soft, workable, austenitic condition, said steel consisting essentially of 14.00% to 21.00% chromium, 2.50% to 8.00% nickel, .70% to 2.50% aluminum, .10%

to .40% nitrogen, carbon 03% maximum, manganese 8.00% maximum, with manganese being inversely proportioned with respect to the nickel content, copper 5.00% maximum, silicon 2.00% maximum, phosphorus 050% maximum, sulphur 050% maximum, and remainder substantially all iron.

2. A chromium-nickel-aluminum stainless steel susceptible to precipitation-hardening by double heat-treatment from a soft, workable, austenitic condition, said steel consisting essentially of 16.5% to 17.5% chromium, 6.0% to 7.5% nickel, 0% to 1.5% aluminum, .10% to .40% nitrogen, carbon 03% maximum, manganese 2% maximum, silicon 1.00% maximum, phosphorus 050% maximum, sulphur 050% maximum, and remainder substantially all iron.

3. A chromiurn-nickcl-aluminum stainless steel susceptible to precipitation-hardening by double heat-treatment from a soft, workable, austenitie condition, said steel consisting essentially of 16.5% to 17.5% chromium, 6.0% to 7.5% nickel, .8% to 1.5% aluminum, .12% to .20% nitrogen, carbon 03% maximum, manganese 1.5% maximum, silicon maximum, phosphorus 050% maximum, sulphur 050% maximum, and remainder substantially all iron.

4. A chromium-nickel-aluminum stainless steel precipitation-hardened by double heat-treatment from the an nealed condition and consisting essentially of 14.00% to 21.00% chromium, 2.50% to 8.00% nickel, .70% to 2.50% aluminum, .10% to .40% nitrogen, carbon 03% maximum, manganese 8.00% maximum and inversely proportioned with respect to the nickel content, copper 5.00% maximum, silicon 2.00% maximum, phosphorus 050% maximum, sulphur 050% maximum, and remainder substantially all iron.

5. A chromium-nickel-aluminum stainless steel precipitation-hardened by double heat-treatment from the annealed condition to give a hardness of at least Rockwell C 40 and an impact strength of at least 6/9 ft. lbs. and consisting essentially of 16.5% to 17.5% chromium, 6.0% to 7.5% nickel, .8% to 1.50% aluminum, .10% to .40% nitrogen, carbon 03% maximum, manganese 1.5% maximum, silicon 1.00% maximum, phosphorus 050% maximum, sulphur 050% maximum, and remainder substantially all iron.

References Cited in the file of this patent UNITED STATES PATENTS Goller May 2, 1950 Walton et al. June 30, 1959 OTHER REFERENCES 

1. A CHROMIUM-NICKEL-ALUMINUM STAINLESS STEEL SUSCEPTIBLE TO PRECIPITATION-HARDENING BY DOUBLE HEAT-TREATMENT FROM A SOFT, WORKABLE, AUSTENITIC CONDITION, SAID STEEL CONSISTING ESSENTIALLY OF 14.00% TO 21.00% CHROMIUM, 2.50% TO 8.00% NICKEL, .70% TO 2.50% ALUMINUM, .10% TO .40% NITROGEN, CARBON .03% MAXIMUM, MANGANESE 8.00% MAXIMUM, WITH MANGANESE BEING INVERSELY PROPORTIONED WITH RESPECT TO THE NICKEL CONTENT, COPPER 5.00% MAXIMUM, SILICON 2.00% MAXIMUM, PHOSPHORUS .050% MAXIMUM, SULPHUR .050% MAXIMUM, AND REMAINDER SUBSTANTIALLY ALL IRON. 